Firstly, similar to government rates you must work for the company in order to get the contracted rate, or prove that you are contracted to the company. Some properties will ask for ID, some may not.

I actually used to work for the Park Hyatt Sydney several years ago, in sales and marketing. Every year corporate rates are negotiated based on the number of room nights from the previous year. I can assure you that the abuse of these codes will not go unnoticed. While a chainwide code may apply, it wouldn't be difficult for IBM to see that rates were being used at a property they may not utilise, such as the Park Hyatt Sydney due to the high average room rate, and realise that the code was being abused. They will also notice a significant spike in room nights booked.

And as mentioned, Hyatt have lurkers on flyertalk, so I have no doubt that the are completely aware that the code is being abused. If you don't have the right to use the discount code, then it is fraud by deception.

Apart from the ethical perspective, the legal one is that you are making reservations in your own name, with address details and credit card details, fully identifying you. Frankly doing so and taking advantage of something you shouldn't is sheer stupidity.